Interleukin-1 is a cytokine having a broad spectrum of biological activities (for reviews, see e.g., Dinarello, C. A. and Wolff, S. M. (1993) New Engl. J. Med. 328:106-113; and Dinarello, C. A. (1993) Trends in Pharmacol. Sci. 14:155-159). IL-1 consists of two structurally related polypeptides, interleukin-1.alpha. (IL-1.beta.) and interleukin-1.beta. (IL-1.beta.). The two forms of IL-1 are encoded by different genes and have only 27-33% amino acid identity but they interact with the same receptor and have similar biological activities. Included among the biological functions attributed to IL-1 are induction of fever, sleep, anorexia and hypotension. IL-1 is also involved in the pathophysiology of inflammatory and autoimmune diseases, including rheumatoid arthritis, septic shock, inflammatory bowel disease and insulin dependent diabetes mellitus. IL-1.alpha. has been specifically implicated in the pathophysiology of psoriasis. IL-1 is also thought to play a role in immune responses to infectious agents and in the pathogenesis of myeloid leukemias.
IL-1.alpha. and IL-1.beta. are both synthesized as approximately 31 kDa precursor molecules that are subsequently processed to a mature form of approximately 17 kDa. IL-1.alpha. and IL-1.beta. differ in that the precursor form of IL-1.alpha. (preIL-1.alpha.) is biologically active and most of the mature IL-1.alpha. (matIL-1.alpha.) remains cell-associated, whereas the precursor form of IL-1.beta. (preIL-1.beta.) must be cleaved to its mature form to become active and the mature form of IL-1.beta. (matIL-1.beta.) is secreted from the cell. Only certain cell types process preIL-1.beta. and secrete matIL-1.beta.. Monocytes and macrophages are the most efficient producers and secretors of IL-1.beta., which is the most abundant form of IL-1 produced upon activation of these cell types.
Interleukin-1.beta. converting enzyme (ICE) is a cytoplasmic cysteine protease required for generating the bioactive form of the interleukin-1.beta. cytokine from its inactive precursor (Black, R. A. et al. (1988) J. Biol. Chem. 263:9437-9442; Kostura, M. J. et al. (1989) Proc. Natl. Acad. Sci. USA 86:5227-5231; Thornberry et al. (1992) Nature 356:768-774; Ceretti, D. P. et al. (1992) Science 256:97-100). ICE is a member of a family of cysteine proteases with shared homology. Other members of this family have been implicated in apoptosis, such as ced-3 (Yuan, J. et al (1993) Cell 75:641-652), Nedd2 (Kumar, S. et al. (1992) Biochem. Biophys. Res. Commun. 185:1155-1161; Kumar, S. et al. (1994) Genes Dev. 8:1613-1626), CPP32 (Fernandes-Alnemri, T. et al. (1994) J. Biol. Chem. 269:30761-30764), Ich-1 (Wang, L. et al. (1994) Cell 78:739-750) and Ich-2 (Kamens, J. et al. (1995) J. Biol Chem. 270:15250-15256; Faucheu, C. et al. (1995) EMBO J. 14:1914-1922). The necessity for ICE in the generation of bioactive IL-1.beta. was demonstrated in mice in which the ICE gene had been functionally disrupted (Li, P. et al. (1995) Cell 80:401-411; Kuida, K. et al. (1995) Science 26:2000-2003). Although these animals are overtly normal, they have a major defect in the production of mature IL-1.beta. after stimulation with lipopolysaccharide.
In vitro studies have demonstrated that ICE cleaves prointerleukin-1.beta. at Asp.sub.116 -Ala.sub.117 to release the fully active 17 kDa form (Black, supra; Kostura, supra). ICE also cleaves prointerleukin-1.beta. at Asp.sub.27 -Ala.sub.2 to release a 28 kDa form. Cleavage at these sites is dependent upon the presence of aspartic acid in the P1 position (Kostura, supra, Howard, A. et al. (1991) J. Immunol. 147:2964-2969; Griffin, P. R. et al. (1991) Int. J. Mass. Spectrom. Ion. Phys. 111:131-149). However, an aspartic acid in the P1 position is not sufficient for ICE specificity. For example, several other proteins containing Asp-X bonds, including prointerleukin-1.alpha., are not cleaved by ICE (Howard, supra).
ICE itself undergoes maturational processing, possibly performed in vivo by ICE itself (Thornberry, N. A. et al. (1992) Nature 356:768-774). Mature ICE is generated from a 404 amino acid precursor protein by proteolytic removal of two fragments, the N-terminal 119 amino acid "pro-domain" and the internal residues 298-316 (Thornberry, supra). Active ICE is therefore composed of two subunits, a 20 kDa subunit (p20) encompassing residues 120 to 297 and a 10 kDa subunit (p10) encompassing residues 317 to 404. The crystal structure of ICE indicates that ICE forms a tetrameric structure consisting of two p20 and two p10 subunits (Walker, N. P. C. et al. (1994), Cell 78:343-352; Wilson, K. P. et al. (1994) Nature 370:270-275). The catalytic amino acid residues of ICE are Cys-285 and His-237. The side chains of four amino acid residues (Arg-179, Gln-283, Arg-341 and Ser-347) form the P1 carboxylate binding pocket (Walker, supra; Wilson, supra).
Because of the apparently harmful role of IL-1 in many disease conditions, therapeutic strategies aimed at reducing the production or action of IL-1 have been proposed. For example, one approach by which to inhibit matIL-1.beta. production and secretion would be to block the activity of ICE with a specific ICE inhibitor. The ability to produce active ICE in vitro is therefore highly desirable to allow for the study of its structure and function. However, in vitro production of ICE can be hampered by the instability of the protein, in particular as a result of autocatalytic degradation which leads to inactive protein.